224 NEBBUDDA DISTRICT. 



to its underlie where it has any dip or hade. And this structure is 

 always obscured, or quite effaced, near the sides of the dyke, where the 

 rock becomes flaky and splits into laminae more or less thin and regular 

 and parallel to the dyke wall. 



Although as a rule, the intrusive rock does not generally much alter 



the beds among which it has come, yet itself is 

 Alteration of dyke. m 



very often influenced near the junction, apparent- 

 ly by the proximity of the mass with which it has come into contact. 

 Fig. 13 b., p. 225, is a diagram representation of a good case of this. A 



fine dyke of greenstone-porphyry exposed in a 

 SoplyeDyke, 



stream, near boplye village, cuts the beds ot the 



Lower Damuda Sandstone. The rock of the dyke is highly crystalline and 

 porphyritic along its centre, but compact and earthy near its sides, and 

 has the structure of sub-vitreous obsidian at the plane of absolute contact 

 with the bedded rock. At a distance of about 3 inches from the sand- 

 stones the porphyritic structure is generally clearly developed, though 

 the change mentioned above is quite gradual, and the passage from the 

 glassy film at the dyke wall to the crystalline rock at its centre, is not 

 everywhere equally rapid. Many small veins start out from the main 

 dyke, and two of these, well seen near the place represented in our sketch, 

 are in part of their course no more than an inch in width, yet along the 

 centre of these little veins, not only is the crystalline structure preserv- 

 ed, but even the porphyritic, and their sides imitate the compact and 

 the pitchy appearance of the corresponding portions of the parent dyke, 

 of which they are in these respects perfect miniatures. It would be 

 imagined that every portion of these small veins must have cooled more 

 rapidly than that portion of the great dyke half an inch from its sides ; 

 still, in the latter case, the rock is invariably compact, and earthy, 

 whereas in the former we find crystals — facts not accounted for by any 

 hypothesis on the subject of crystallization from fusion with which I am 

 acquainted. 



